1. Field of the Invention
This invention relates to an optical pickup device and an optical disc apparatus for enabling recording of information signals on a plurality of types of the optical recording mediums having different layered structures by differences in the substrate thickness and materials and for enabling reproduction of the recorded information signals.
2. Description of the Related Art
As an optical recording medium, a compact disc (CD), which is a read-only optical disc 120 mm in diameter, has been proposed. This CD 100 has its disc substrate (transparent layer) 101 molded of a transparent polycarbonate resin having high light transmittance, high mechanical resistance and resistance against chemicals, or of transparent synthetic resin materials, such as polyvinyl chloride resin or acrylic resin, as shown partially in FIG. 1a. On one of the major surfaces of the disc substrate 101 are formed pits arranged on concentric circles by transcription by a stamper built into a molding metal mold. These pits 102 are formed to form a recording track by being formed as tiny encoded holes having different circumferential lengths in association with pre-set information signals. The CD 100 has a reflective layer 103 formed by evaporation of aluminum of high light reflectance on a pit forming surface of the disc substrate 101. A protective layer 104 is then applied for completing the CD 100.
In the CD 100, the information signals recorded as pits 102 on the disc substrate 101 are reproduced by an optical pickup device 200 constructed as shown in FIG.2.
Referring to FIG.2, the optical pickup device 200 is made up of a semiconductor device 201, a grating 202, a beam splitter 203, a collimator lens 204, an objective lens 205 and a photodetector 207.
In the above-described optical pickup device 200, a light beam radiated from the semiconductor device 201 is split by the grating 200 into a main beam and a side beam. These beams are reflected by a reflecting surface 203a of the beam splitter 203 and collimated by the collimator lens 204 into a collimated beam which is then converged by the objective lens 205 on a point of the signal recording surface of the CD 100.
The return light beam, reflected back from the signal recording surface of the CD 100, again falls via the objective lens 206 and the collimator lens 204 on the beam splitter 203. The return light beam is transmitted through the beam splitter 203 to fall on the light receiving section of the photodetector 206.
The information recorded on the signal recording surface of the CD 100 is reproduced in this manner based on the detection signals outputted by the light receiving section of the photodetector 206.
Recently, attempts are being made towards raising the recording density of the optical disc as a subsidiary storage device for a computer or as a package medium for speech or video information. For raising the recording density, there is known a method of increasing the numerical aperture NA of the objective lens beyond the numerical aperture of the objective lens of the optical pickup for a conventional compact disc and reducing the beam spot diameter by employing a short wavelength light source. However, if the numerical aperture NA is increased, the tolerable range for the tilt of the optical disc is diminished.
On the other hand, since the signal recording surface of the optical disc is provided via a transparent substrate having a pre-set substrate thickness, which is usually 1.2 mm in the case of a compact disc, wavefront aberration is produced if the optical disc is tilted relative to the optical axis of the objective lens of the optical pickup, thus affecting RF signals (playback signals). As for the wavefront aberration, three-order coma aberration, proportionate to a third power of the numerical aperture and approximately first power of the skew angle .theta. and inversely proportionate to the wavelength, is predominant.
The optical disc, having the transparent substrate of, for example, polycarbonate, mass-produced at lower cost, has the skew angle of, for example, as much as .+-.0.5 to .+-.1.degree.. Thus the light spot from the semiconductor laser device of the optical pickup device, converged on the optical disc, becomes non-symmetrical by the wavefront aberration, thus increasing the inter-symbol interference, thereby disabling correct reproduction of the RF signals.
Since the three-order coma aberration is proportionate to the thickness of the disc substrate, as described above, the disc substrate thickness may be set to, for example, 0.6 mm, for significantly decreasing the three-order coma aberration.
In this case, there exist two optical disc standards having different characteristics, that is a standard having a thicker disc substrate (for example, 1.2 mm) and a standard having a thinner disc substrate (for example, 0.6 mm).
If a plan-parallel plate with a thickness equal to t is insert into an optical path, there is produced spherical aberration proportionate to t.times.NA4. Therefore, the objective lens is designed for canceling this spherical aberration.
Meanwhile, the spherical aberration differs with disc substrate thicknesses, so that, if desired to reproduce an optical disc conforming to a given standard, such as a compact disc, write-once optical disc or a magneto-optical disc with the disc substrate thickness of 1.2 mm using an objective lens conforming to the optical disc having a disc substrate thickness of 0.6 mm conforming to the other standard, there is produced the spherical aberration due to difference in the disc substrate thickness, thus significantly exceeding the range of the error in the disc substrate thickness accommodated by the optical disc. Thus the signals cannot be detected correctly from the return light from the optical disc, with the result that the optical discs of the plural types with different disc substrate thicknesses cannot be reproduced by the conventional optical pickup.
Thus, there has also been proposed such a system in which plural objective lenses designed to cancel the spherical aberration of the plural disc types are provided and an objective lens conforming to the type of the optical disc to be reproduced is inserted into the light path depending on the disc types to be reproduced for coping with plural disc types with different disc substrate thicknesses.
However, in this objective lens switching type optical pickup, the beam spot diameter is reduced using a shorter light source wavelength of the order of 635 to 650 nm for coping with high-density optical discs (DVDs).
The high-density optical discs (DVDs) are required to have a function of enabling reproduction of the CD 100. In the DVD reproducing device, an objective lens 107 for DVD, optimized for a DVD 105 having an information signal recording layer at a distance of 0.6 mm from the major surface of the disc substrate, is used, as shown in FIG. 3a. However, since the CD 100 has a signal recording layer at a distance of 1.2 mm from the major surface of the disc substrate, the laser light spot cannot be converged optimally by the objective lens for DVD 107.
Thus the reproducing device with CD-DVD compatibility is required to cause the laser light to be converged on a signal recording layer at two positions of 0.6 mm and 1.2 mm from the disc substrate surface. As this type of the reproducing device with CD-DVD compatibility, there have been proposed an axial sliding type optical pickup device in which an objective lens for CD and an objective lens for DVD are mounted and mechanically switched, and a two focal point type optical pickup device in which part of the laser light is diffracted by a hologram formed on the surface of an objective lens and the diffracted laser light and the non-diffracted laser light form two focal points for the CD 100 and for the DVD 105. There is also known another reproducing device with CD-DVD compatibility in which a liquid crystal shutter 109 is provided in the laser light path of the DVD reproducing device for interrupting the peripheral light for converging the laser light for varying the focal length of a sole objective lens 107 into one for CD 100 and one for DVD 105 as shown in FIG. 4.
The optical recording medium may not only be an above-mentioned read-only CD 100 but also be a compact disc recordable CD-R 110 capable of recording the information only once. The CD-R 110 is similar to the CD 100 in physical properties, such as diameter, weight or thickness, or recording capacity, however, it can be produced in smaller quantities economically and is more durable than the CD 100 so that it is suited for data storage. This CD-R 110 has a transparent polycarbonate substrate 111 on which an organic dye layer 112, a gold reflective layer 113 and a protective layer 114 are layered in this order, as shown in part in FIG. 1b. In the CD-R 110 is also formed a laser light illumination guide (groove) covered by the organic dye layer 112. The organic dye layer 112 is reacted with polycarbonate of the substrate under the heat of the laser light for forming pits 115 corresponding to the illuminated laser light for after-recording the information signals.
Since the organic dye is used in the CD-R 110 as a film constituting a signal recording surface, the CD-R has high light absorbance for the wavelength of the order of 635 to 650 nm. Thus the reflected light from the recording pit on the signal recording surface of the optical disc is not of sufficient intensity.
That is, with the usual CD 100, in which an aluminum evaporated film is used as a reflective surface of the signal recording surface, it is difficult to detect changes in light volume of the reflected light from the optical disc, such that it is practically impossible to reproduce the CD-R.
Thus it may be envisaged to load both an optical pickup coping with shorter wavelengths and another optical pickup coping with shorter wavelengths on the optical disc device. However, if plural optical pickup devices are used, the manufacturing cost is increased, while the optical disc apparatus is correspondingly increased in size.